The present invention relates to a gas laser oscillator having an optical axis that is matched with the axial direction of the discharge tube, and more particularly to a gas laser oscillator capable of obtaining a laser beam of high quality.
FIG. 4 is a schematic block diagram of a conventional gas laser oscillator. In FIG. 4, reference numeral 1 is a discharge tube made of glass or other dielectric material, and the inside of the discharge tube 1 is filled with laser gas, or laser gas is circulated by a gas circulating apparatus not shown in the drawing. Reference numerals 2 and 3 are electrodes disposed at both ends of the discharge tube 1, reference numeral 4 is a high voltage power source connected to the electrode 2 and electrode 3, and reference numeral 5 is a discharge space inside the discharge tube 1 lying between the electrode 2 and electrode 3. Reference numeral 6 is a fully reflective mirror disposed toward one opening of the discharge space 5, and reference numeral 7 is a partially reflective mirror disposed toward the other opening of the discharge space 5, and the fully reflective mirror 6 and partially reflective mirror 7 form an optical resonator. Reference numeral 8 is a laser beam emitted from the partially reflective mirror 7.
In the conventional gas laser oscillator, the operation is described below. Discharge occurs in the discharge space 5 between the electrode 2 and electrode 3 connected to the high voltage power source 4. By this discharge, the laser gas in the discharge space 5 is excited by the discharge energy. The excited laser gas is set in a state of resonance by the optical resonator formed by the fully reflective mirror 6 and partially reflective mirror 7, and being optically amplified by this resonance, the laser beam 8 is issued from the partially reflective mirror 7. This laser beam 8 is used in various applications of laser processing.
FIG. 5(a) and FIG. 5(b) are diagrams for explaining the operation of the optical resonator in the gas laser oscillator, showing more specifically the structure of the gas laser oscillator. In FIG. 4, only one discharge tube is shown, but generally, as shown in FIG. 5(a) and FIG. 5(b), plural discharge tubes 1 are disposed in series along the optical axis. Although mere cylindrical forms are expressed in FIG. 5(a) and FIG. 5(b), same as the discharge tube 1 in FIG. 4, an electrode 2 and an electrode 3 are disposed at both ends of each discharge tube 1, and a high voltage power source 4 is connected between each pair of electrodes, that is, electrode 2 and electrode 3, and a discharge space is formed inside of each discharge tube 1.
In the gas laser oscillator shown in FIG. 5(a) and FIG. 5(b), when discharge occurs in the discharge space 5, a standing wave 10 is formed. The property of this standing wave 10 is determined by the size of the resonance space 9 and the curvature of the fully reflective mirror 6 and partially reflective mirror 7. This property of standing wave is known as TEM (transverse electromagnetic) mode order. Generally the lower the TEM mode order, the better is the laser beam converging, and it is known that higher processing performance is obtained. For example, the smaller the inside diameter of the discharge tube 1, the narrower is the resonance space, and therefore oscillation of high-order TEM mode is suppressed, the TEM mode order becomes lower and light converging is enhanced, so that a laser beam of high processing performance is obtained.
On the other hand, in the gas laser oscillator having thus explained construction, of the electric energy supplied from the high voltage power source 4, all energy excluding the portion converted into the laser beam 8 becomes heat. Therefore, to maintain the parallelism between the fully reflective mirror 6 and partially reflective mirror 7 by preventing deformation due to this generated heat, it is necessary to cool the fully reflective mirror 6 and partially reflective mirror 7 and the peripheral parts supporting them.
Concerning cooling of the fully reflective mirror 6 and partially reflective mirror 7 and their peripheral parts in the conventional gas laser oscillator, as disclosed in Japanese Laid-open Patent No. 56-90588, the construction being shown in FIG. 8. As shown in FIG. 8, the fully reflective mirror 6 and partially reflective mirror 7 for resonance are respectively held by a flange 31 and a flange 32. By coupling these flanges 31 and 32 through a support element 33, the parallelism of the fully reflective mirror 6 and partially reflective mirror 7 necessary for laser oscillation is maintained. A passage 35 is provided inside the support element 33, and it is intended to cool by passing oil or other cooling medium in this passage 35. In the conventional gas laser oscillator, the passage 35 of the cooling medium inside the support element 33 was straight from the inlet to the outlet of the cooling medium.
The high voltage power source 4 is, as shown in FIG. 15, composed of a switching power source 44, a step-up transformer 45, and a rectifying and smoothing circuit 46. Generally, the gas laser oscillator is composed of plural discharge tubes, and each discharge tube requires the step-up transformer 45 and rectifying and smoothing circuit 46. In one switching power source 44, the primary side of plural step-up transformers 45 can be connected, and therefore only one switching power source 44 is enough for plural discharge tubes.
The step-up transformer 45 is composed of a step-up transformer main body 49 and a transformer container 47 as shown in FIG. 16, and the transformer container 47 is filled with insulating oil 48, and the step-up transformer main body 49 composed of coil and core is immersed in the insulating oil 48. A top plate 50 is disposed in the upper part of the transformer container 47, and an oil feed port 51 provided in the top plate 50 is sealed with an oil cap 52 except when feeding oil, so that the entire step-up transformer 44 is in a sealed structure.
The conventional gas laser oscillator thus constructed had several problems.
First, to lower the TEM mode order, in the discharge tube 1 shown in FIG. 5(a), when the inside diameter of the discharge tube 1 is reduced as shown in FIG. 5(b), scattered beam 8a is likely to occur in the resonance space 9, and scattered beam 8a mixes into the laser output. FIG. 6 shows an output mode in a conventional gas laser oscillator. The axis of abscissas in FIG. 6 denotes the distance toward outside from the center of the output laser beam, and position 0 indicates the center. The axis of ordinates represents the energy density of the laser beam. FIG. 6 shows that scattered beam 8a is present in the peripheral region A of the laser beam 8. Laser cutting by using such a laser beam causes an increase in the thermal effects around the cut section due to the scattered beam 8a included in the peripheral region, and lowers the cutting quality. As explained above, when attempting to improve the light converging and enhance the processing performance by lowering the TEM mode order, the scattered beam mixes into the output laser beam to increase the thermal effect range, which leads to a first problem of deterioration of processing quality.
As mentioned herein, in the gas laser oscillator, of the electric energy supplied from the high voltage power source 4, all energy except for the portion converted into the laser beam becomes heat 36. Such heat 36 was dissipated, conducting to the parts composing the gas laser oscillator, such as flanges 31 and 32 existing around the resonance space 9 or the support element 33 for coupling them, through the laser gas filling the resonance space 9 as shown in FIG. 9.
The support element 33 is a member for maintaining the parallelism between the fully reflective mirror 6 and partially reflective mirror 7, and when uniformity of temperature distribution in the support element 33 is lost due to the conducting heat 36, the support element 33 is thermally deformed, and accurate parallelism between the fully reflective mirror 6 and partially reflective mirror 7 cannot be maintained. To avoid this inconvenience, it was designed to cool by passing a cooling medium in the support element 33. However, in the conventional gas laser oscillator, the passage 35 of the cooling medium was straight from the inlet to the outlet of the cooling medium inside the support element 33. Accordingly, heat convection occurs in the cooling medium inside the passage 35, and temperature distribution of the cooling medium itself is not uniform. Due to heat convection of the cooling medium itself, the temperature is higher in the upper part and the temperature is lower in the lower part of the support element 33, and the temperature distribution is uneven, and thermal distortion occurs. This thermal distortion leads to a second problem of making it difficult to maintain the accurate parallelism between the fully reflective mirror 6 and partially reflective mirror 7.
In the step-up transformer 45 of the conventional high voltage power source, the step-up transformer main body 49 was contained in the transformer container 47, and the transformer container 47 was in a sealed structure. Due to the heat generated in the step-up transformer main body 49, the temperature of the insulating oil 48, in which the transformer main body 49 is immersed, and the air 53 in the transformer container 47 are raised. When the transformer container 47 is enclosed by the top plate 50 and oil cap 52, the internal atmospheric pressure in the transformer container is raised, and a pressure difference occurs between the inside and outside of the transformer container 47. This pressure difference causes the insulating oil 48 to leak out of the transformer container 47.
To eliminate the pressure difference between the inside and outside of the transformer container 47, as shown in FIG. 17, a penetration hole was provided in the oil cap 52. As a result, occurrence of a pressure difference between the inside and outside of the transformer container 47 could be prevented, but the insulating oil 48 splashed up and leaked during transportation. FIG. 18 and FIG. 19 are modified examples of the penetration hole provided in the oil cap 22, but it was a third problem that leakage of the insulating oil 48 could not be prevented completely.